The present invention relates to a random polyimide copolymer. More particularly, the invention relates to a random polyimide copolymer which is wet-etchable in its poly(amic acid) or partially cured poly(amic acid) precursor form.
Multichip module (MCM) packaging structures are being developed to achieve higher performance through component miniaturization. Multiple chips are mounted in close proximity and interconnected by a series of multilayer patterned conductor pathways insulated by a polymeric, preferably polyimide, dielectric. The structure is usually built on a suitable substrate such as a cofired multilayer ceramic. The package provides greater chip density and reduced interconnect length resulting in shortened signal propagation times and reduced power consumption. Design requirements on the interlayer dielectrics used in MCM fabrication are also becoming increasingly stringent as circuit density and the number of signal layers increases. Polyimide materials that combine low dielectric constant, water absorption and thermal expansion coefficient with high toughness, thermal stability, glass transition temperature, solvent resistance and adhesion are desirable for future generation MCM dielectrics.
Traditionally, circuit patterns have been defined in the fully cured polyimide dielectric layers by dry or plasma etching processes. While these techniques are useful for resolving fine geometries in thick films, the materials and processes involved are often expensive and/or time consuming. More recently, polyimide precursors that are inherently photosensitive have offered an alternative method for resolving feature sizes suitable for high density interconnect (HDI) applications. However, when overall cost becomes critical and the features to be resolved in the dielectric layers are consistent with those encountered in low and medium density interconnect (LDI and MDI respectively) applications, traditional wet etching processes may be desirable for patterning the MCM polyimide interlayer dielectric in its poly(amic acid) form.
U.S. Pat. No. 4,778,872, Sasaki et al, issued on Oct. 18, 1988, discloses an aromatic poly(amic acid) derived from 15 to 85 mole % of 3,3', 4,4'-biphenyltetracarboxylic dianhydride, 15 to 85 mole % of pyromellitic dianhydride, 30 to 100 mole % of p-phenylene diamine and 0 to 70 mole % of 4,4'-diaminodiphenylether. The aromatic poly(amic acid) can be formed into an aromatic polyimide film having a low thermal expansion coefficient, high mechanical strength and good flexibility. Sasaki et al do not disclose using flexible aromatic anhydrides and aromatic diamines as monomer components to impart wet-etchability to the poly(amic acid).
U.S. Pat. No. 4,623,563, Noda et al, issued on Nov. 18, 1986, discloses a polyimide/metallic foil composite film wherein the polyimide comprises at least 70 mole % of 3,3', 4,4'-biphenyltetracarboxylic dianhydride, 30 mole % or less of pyromellitic dianhydride, 80 mole % or more of p-phenylenediamine and 20 mole % or less of another diamine such as 4,4'-diaminodiphenylether or m-phenylenediamine. Noda et al require at least 80 mole % of the rigid p-phenylene diamine component and at least 70 mole % of the rigid 3,3', 4,4'-biphenyltetracarboxylic dianhydride in their copolyimides in order to match the CTE of the polyimide to that of the metallic foil, thereby minimizing curling of the composite film. Moreover, there is no disclosure of introducing additional flexible dianhydrides and/or diamines to impart wet-etchability to the poly(amic acid).
European Patent Application 0,470,446, published on Feb. 12, 1992, discloses polyimides having improved glass transition temperature for use in fiber filled composite articles comprising a mixture of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and either 3,3', 4,4'-biphenyltetracarboxylic dianhydride or oxydiphthalic dianhydride and aromatic diamines such as m-phenylene diamine, p-phenylene diamine and 4,4'-diaminodiphenylether. There is no disclosure of using flexible dianhydrides and/or diamines as monomer components to impart wet-etchability to the poly(amic acid).
Accordingly, a need persists for a high performance polyimide for MCM fabrication having a good balance of mechanical, thermal and electrical properties and having the advantage of wet-etchability in its poly(amic acid) precursor form over a broad process range using conventional aqueous developers.